In mechanical watches, regulating members, in particular escapements have to satisfy several “safety” criteria. One of the safety devices, the anti-trip system, is designed to prevent the angular extension of the balance beyond a normal angle of rotation.
The technical problem is to devise a safety mechanism, particularly an anti-trip system, which limits the angle of pivoting of a balance during excessive accelerations, in particular in the event of shocks, notably for a detent escapement. The anti-trip mechanism must be capable of acting in both directions of pivoting of the balance, i.e. both during extension and contraction of the balance spring.
One solution consists in changing the geometry of the balance spring by causing the lugs of consecutive coils to cooperate in abutment, so as to render some coils inactive and thus to modify the rigidity of the balance spring and its response to impulses. A mechanism of this type, capable of limiting the angular travel of the balance in both directions of pivoting is known from EP Patent No 2 434 353 A1 in the name of Montres Breguet SA, which discloses an anti-trip balance spring wherein notches pertaining to consecutive coils cooperate with each other, both during the contraction and the expansion of the balance spring. The manoeuvre is efficient, however the torque pick up remains relatively abrupt.
It is an object of the present invention to improve safety, while only very slightly disturbing the inertia of the balance, by limiting the angular travel of the balance in both directions of rotation, and gradually picking up torque.